Enhanced API and added a spec for ORMultiMap

In addition the removeBindings api call has been removed in favour of using the newly introduced get, put and remove map level operations.
My rationale here is to allow `ORSet` to be used in exactly the same manner as `ReplicatedData` for `ORMap`. With `ORMap`, you can certainly `put` any `ReplicatedData` and lose its history. So the problem that you are describing must surely exist for `ORMap`, right?

My rationale for `get` and `put` in addition to the binding methods is so that operations on the entire `ORSet` may be completed - including filtering. We have this need and you'll see that the `removeBindings` method has been removed so that we don't start polluting this class with methods that should otherwise be available to `ORSet`.